The present invention generally regards the bonding of wafers to create microelectromechanical systems. More particularly, the present invention regards a method and system for bonding a silicon wafer to a cap wafer using a laser.
Sensor components may be manufactured using surface micromechanics, deposition, or etching technologies. Microelectromechanical systems (MEMS) are used for a variety of devices including oscillators, channels, pumps, accelerometers, and filters. To protect against external interference during operation, it may be desirable to hermetically seal the MEMS sensor structure with a cap. This sealing process may take place in a vacuum or under controlled atmospheric conditions. Capping is usually performed on the wafer level using a structured silicon or glass wafer. This cap wafer is designed so that it can be placed on the sensor wafer in an aligned manner.
Glass and silicon wafers have been bonded using anodic bonding techniques. Anodic bonding uses borate glass containing alkali ions (e.g., Borofloat, Pyrex) as the cap and a conductive or semi-conductive material as the bond frame (e.g. silicon). Anodic bonding requires good contact between the surface layers, and therefore requires smooth contact surfaces. Some mechanical compression is also required to provide a good contact between the layers. High temperatures, on the order of 300 to 500 degrees Celsius, and high voltages are required for successful anodic bonding. A charge is applied between the glass and the silicon to create a bond between the atoms at the interface between the different layers. One important disadvantage of anodic bonding is that thermally induced mechanical stress results from the heating of the different layers since the SI wafer and the glass wafer have different expansion coefficients. Another disadvantage of anodic bonding is that, due to the high electrostatic fields, the movable MEMS structures may stick during the bonding process or may have to be electrically insulated from the electrostatic fields.
Alternatively, glass frit bonding has been used to bond the cap wafer to the Si sensor wafer in a stable manner. Wafer bonding techniques such as glass frit bonding use special glasses that contain PbO and that soften at moderate temperatures. Glass frit bonding has the advantage that the topography of the contact surface and particles at the bonding interface have little effect. The disadvantages of glass frit bonding include crimping of the solder, the wide bonding interface required, the offset between the cap and wafer, outgassing from the bonding process, and the difficulty of further processing the object.
Another alternative bonding technique is silicon direct bonding, which may have the disadvantages that the topography of the contact surfaces must be very smooth and that any particles present at the contact surface weaken bond adhesion. Additionally, silicon direct bonding may require very high temperatures on the order of 700 to 1000 degrees Celsius.
Other bonding techniques are eutectic bonding or adhesion techniques such as gluing. These techniques are used for various applications. Major disadvantages of gluing include outgassing, poor alignment, poor durability, a large bonding area, and the difficulty of further processing the object.
Some important considerations when selecting the bonding technique are thermal mismatch between the sensor wafer, the cap wafer and the bonding medium used; the effect of topography and particles; the further processability of the combination; the bonding surface required for a stable bond; the emission of gases from the bonding medium; and special measures such as electrical shielding and the like. Therefore, there is a need for a method of consistently and reliably bonding a sensor wafer to a cap wafer to provide a hermetically sealed cap. Also desirable is a method of hermetically sealing a cap wafer to a silicon wafer that requires only a small area for the bonding area.
The use of lasers in manufacturing is spreading due to the abundance of different types of lasers that are available economically. For instance, aluminum may be welded during airplane production using localized laser energy.
Channels may be constructed as part of the production of Bio-MEMS (biological microelectromechanical systems). These channels may be used for the passage of fluids. In some bonding methods, the channel produced may have cavities at the interface between different layers. These cavities may allow dirt or impurities to corrupt a biological sample or distort a sensor measurement. Therefore, a method of bonding to produce a channel with rounded corners for use in Bio-MEMS or other applications may be desirable.
A system and method for bonding a glass wafer to a silicon wafer is provided that uses localized laser energy. In this method, a transparent cap wafer (e.g. glass or plastic) is transilluminated by a laser beam having a high power density while the cap wafer is aligned and in close contact with a silicon wafer. Only the cap wafer is transparent to the laser beam so that almost all the light is absorbed in the silicon wafer. As a result, the glass boundary layer melts in a localized manner and bonds with the silicon wafer. In one preferred focusing and guidance system, the laser is moved in discrete steps over the wafer such that narrow bonding traces (2 xcexcm-100 xcexcm in width) are formed from molten silicon in the bonding surfaces, and these traces hermetically enclose the sensor structure.
The method for creating a bond between a wafer and a cap includes providing the wafer with at least one microelectromechanical system and a bond frame arranged on the wafer. The bond frame is arranged on an outer perimeter of the wafer with respect to the at least one microelectromechanical system and has a high absorption coefficient with respect to a laser beam wavelength. A cap with a low absorption coefficient with respect to the laser beam wavelength is arranged on top of the wafer. The laser beam is projected through the cap and impinges on the bond frame thereby heating the bond frame. A portion of the cap adjacent to the bond frame is melted creating a bond between the cap and the bond frame.
A device is provided including a wafer, at least one microelectromechanical system, a bond frame, and a cap. The cap is bonded to the bond frame by a laser beam projected through the cap and impinging on the bond frame and heating the bond frame. The heat from the bond frame melts the cap which forms a bond with the bond frame.
A system for bonding a wafer to a cap is provided with a holder which applies a normal force to the wafer and the cap and which has an aperture for the laser beam. Also included is a laser directed at the aperture and projecting at a frequency that passes through the aperture with about zero absorption. The laser beam passes through the cap and impinges on the wafer, thereby heating the wafer and causing the cap to bond to the wafer.